1. Field of the Invention
The present invention relates to image forming apparatuses such as copiers, printers, recorded image displaying apparatuses, and facsimile machines, which form visible images by electrophotographic methods. In particular, the present invention relates to an image forming apparatus which performs image formation by using a rotary developing device rotatably holding a transparence developing unit and a color developing unit.
2. Description of the Related Art
Nowadays, in multifunction apparatuses and facsimiles having copying and printing functions, electrophotographic image forming apparatuses are widely used. In particular, electrophotographic full-color image forming apparatuses are popular.
Reproduction of color images with such full-color image forming apparatuses is promoted. Hence, it is desired to increase image quality, for example, by increasing a color reproduction range and reducing graininess of an image.
Frequently, gloss is entirely applied to an image, so that the image is finished as a photographic image. Also, variation of gloss is applied to an image, so that a new expression not relying on color is applied to the image. Printed products obtained in such manner are common.
Some configurations have been suggested to realize such an expression with an electrophotographic image forming apparatus. The configurations include an image forming method using transparent toner.
For example, Japanese Patent Laid-Open No. 2006-251717 describes a configuration having a developing unit capable of forming a transparent toner image in addition to four color developing units capable of forming four-color toner images. This patent also describes a configuration in which a plurality of image forming portions including a plurality of photosensitive members and corresponding developing units are arranged in a moving direction of an intermediate transfer member (tandem configuration). The patent further describes a configuration in which a photosensitive member is provided and a plurality of developing units are rotationally moved relative to a development position (rotary configuration). The rotary configuration is desirable because the size of the image forming apparatus is easily reduced as compared with the tandem configuration.
On the other hand, the color toner is more likely mixed into the developing unit using the transparent toner than in the case of the developing unit using the color toner. This is because the mixed color is apparent even though only a small amount of the color toner is mixed into the transparent toner.
The inventor has found that such toner mixture may be caused by the following factor in addition to a case in which the color toner is scattered and floated in the image forming apparatus and then mixed into the transparent toner. In particular, assuming that the rotary configuration is employed for the developing units, when the developing units are rotationally moved, development sleeves collect the toner which accumulates after being scattered in the image forming apparatus. In the rotary developing units, in some cases, a toner receiver is provided below a rotary unit which holds and rotates the developing units. The toner receiver receives the toner falling from the developing unit, and the toner accumulates therein. When the accumulated amount of the toner increases such that a moving locus of the development sleeve during rotation of the rotary unit interferes with the accumulated toner, the development sleeve collects the toner accumulated in the toner receiver. Hence, toner of one color may mix with toner of other colors.
The inventor has found that the toner mixture caused by the toner accumulated in the toner receiver is more noticeable in a certain developing unit in the apparatus in which the rotary unit is moved to a reference position (home position) for rotation start every image formation operation. The developing unit is one that firstly reaches the toner accumulated portion after the rotary unit is located at the home position and then rotation of the rotary unit is started. While the rotary unit is rotated, the development sleeves of the rotated developing units collect the toner that accumulates in the toner receiver by a small amount at a time. Thus, the collecting amounts may become substantially uniform among the developing unit. However, when the rotary unit is located at the home position and is in a standby state, the developing units do not collect the toner. Hence, the accumulated amount of the falling toner may increase. Then, when the rotation of the rotary unit is started, the developing unit which firstly reaches the position collects a large amount of toner. Thus, the toner mixture phenomenon occurs.
The toner accumulated in the toner receiver causes the toner to be mixed by a larger amount than the toner floating in the apparatus does. In the developing unit using the transparent toner, the toner mixture should be particularly considered.